1955 - Pakistan joins the South East Asia Treaty Organization and Central Treaty Organization — two Western regional defense pacts. Between 1953 and 1961, Pakistan receives some $2 billion in U.S. aid, a quarter of that in military assistance.

1960 - Pakistan allows the United States to fly its spy planes from an air base on the outskirts of Pakistan’s northwestern city of Peshawar for reconnaissance of the Soviet Union. A U.S. U-2 spy plane flown from this air base was shot down by the Soviet Union over its air space on May 1, worsening relations between Pakistan and the Soviet Union. Pakistan publicly claimed to have been deceived by the United States about the use of the base.

1962 - Indo-China war prompts U.S. President John F. Kennedy to offer India economic and military aid. Pakistani President Mohammad Ayub Khan expresses displeasure over not having been consulted beforehand, as Kennedy had promised.

1965 - Second Indo-Pakistan war prompts U.S. to suspend military assistance to both sides, leading to a cooling of U.S.-Pakistani ties.

1970 - Pakistan plays a behind-the-scenes role to open communications between its old ally China and the United States. These efforts result in a secret visit of then-U.S. Secretary of State Henry Kissinger to China in 1971 and then by President Richard Nixon the following year, the first U.S. presidential trip to China.

1971 - Civil war between West and East Pakistan leads to the third Indo-Pakistan war. East Pakistan breaks away to form Bangladesh. U.S. again suspends military aid. Many in Pakistan begin to see United States as an unreliable ally.

1977 - Army chief General Mohammad Zia-ul-Haq stages a coup, overthrowing the government of Prime Minister Zulfikar Ali Bhutto.

1979 - President Jimmy Carter’s administration cuts off military aid to Pakistan again over its covert construction of a uranium enrichment facility.

November 1979 - Enraged Pakistani students burn the U.S. Embassy in Islamabad on rumors that U.S. forces have attacked Islam’s holiest city of Mecca in Saudi Arabia. Two U.S. Marines and two Pakistanis are killed in the incident.

December 1979 - The Soviet Union invades Afghanistan. The United States begins to view Pakistan as a front-line state in the effort to stop Soviet expansionism.

September 1981 - President Ronald Reagan’s administration negotiates a five-year, $3.2 billion economic and military aid package with Islamabad. Pakistan becomes the main route for arms and supplies for the Afghan resistance.

1985 - Pressler amendment added to the Foreign Assistance Act. It requires the president to certify to Congress that Pakistan does not possess a nuclear device as a condition for receiving aid.

1988 - Soviet Union withdraws from Afghanistan. U.S. begins to take a harder look at Pakistan’s nuclear activity.

1990 - President George H.W. Bush suspends aid to Pakistan under the Pressler amendment, cutting all military funds and most economic assistance. Deliveries of major military equipment are also suspended. Washington refuses to deliver 71 F-16 fighters ordered by Pakistan the previous year.

1990s - U.S. disengagement from Pakistan and Afghanistan in the 1990s causes growing resentment in Pakistan. Many people today feel the current situation is the result of the U.S. failure to remain engaged.

1998 - India conducts a nuclear test and Pakistan follows with its own test. Both declare themselves nuclear-weapons states. The U.S. imposes sanctions, restricting military sales, economic assistance and loans to the Pakistani government.

1998 - U.S. agrees to pay Pakistan $324.6 million to settle legal disputes involving the U.S. government over the refusal to deliver the 71 F-16 fighters. Washington also gives Pakistan $140 million worth of surplus wheat and soy, but resentment over the failed arms deal lingers.

1999 - After meeting U.S. President Bill Clinton in Washington, Pakistani Prime Minister Nawaz Sharif agrees to withdraw Pakistani troops and anti-India militants from the Kargil area of Indian Kashmir after a bloody conflict that threatened to go nuclear.

1999 - Army Chief General Pervez Musharraf ousts elected government in a coup. Clinton says he hopes Pakistan will return to democracy as soon as possible.

2001 - Musharraf turns into President George W. Bush’s key ally after the Sept 11, 2001 attacks on New York and Washington. Pakistan officially abandons support for the Taliban regime in Afghanistan and joins U.S.-led war on terrorism. U.S. aid to Pakistan soars. Between 2002 and 2008, U.S. provides nearly $12 billion in aid to Islamabad, about three-quarters of it military assistance.

2004 - A.Q. Khan, founder of Pakistan’s nuclear program, confesses to supplying nuclear-weapons technology to North Korea, Iran and Libya. Washington presses for his arrest, but Khan, a national hero, is confined to his home instead.

2005 - U.S. responds to Kashmir earthquake with $510 million commitment to relief, reconstruction and humanitarian assistance.

2007 - Musharraf steps down as army chief amid plans to hold elections to return the country to democracy, but he retains the presidency. Former Prime Minister Benazir Bhutto is assassinated during political campaign.

2008 - Elections bring Bhutto’s widower, Asif Ali Zardari, to power as prime minister at the head of a coalition government. Pressure from the new government forces Musharraf to step down as president.

2009 - Newly elected U.S. President Barack Obama names Richard Holbrooke as special envoy for Afghanistan and Pakistan. He later unveils a new strategy to “disrupt, defeat and dismantle” al Qaeda and its safe havens in Pakistan. U.S. approves $7.5 billion in aid for Pakistan over the next five years.

2010 - Failed car bombing on May 1 in New York’s Times Square leads to the arrest of Pakistani-American Faisal Shahzad. Pakistani Taliban claims responsibility for the attempt. U.S. warns of “severe consequences” if a successful attack on U.S. soil is traced back to Pakistan.

Sources: Congressional Research Service, Council on Foreign Relations, U.S. National Security Archives

Reporting by David Alexander and Chris Allbritton; editing by Todd Eastham